EP260329b — All Circulars

A. Volnova (IKI), E. Klunko (ISTP), D. Anarbek (FAI), M. Krugov (FAI), A.
Pozanenko (IKI), N. Pankov (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of the fast X-ray transient EP260329b (Song et al.,
GCN 44142; EP Team, GCN 44155) with the AZT-33IK 1.5m telescope of the
Sayan Solar Observatory (Mondy) taking several 40-second expositions in
R-band, and with the AZT-20 telescope of Assy-Turgen observatory taking
several 30-second exposures in r' filter on Mar. 29. In the stacked frames
we do not detect the optical counterpart reported previously (Li et al.,
GCN 44139; Zhu et al., GCN 44140; Eyles-Ferris et al., GCN 44144; Aryan et
al., GCN 44147; Lipunov et al., GCN 44150; Mo et al., GCN 44163; O'Neill et
al., GCN 44169).
Preliminary photometry and observational details are the following:

Date        UT start  t-T0      Exp.    Filter Obj.   Err.  UL
 Site/Telescope
                     (mid,days) (n*s)                      (3sigma)
2026-03-29  15:24:19  0.22110   90*40   R      n/d    n/d   21.1
 Mondy/AZT-33IK
2026-03-29  19:57:04  0.40444   80*30   r      n/d    n/d   21.6
 Assy-Turgen/AZT-20

The photometry is based on several nearby stars from the SDSS-DR16
catalogue (Lupton transformations were used for the R-band values) and is
not corrected for the Galactic extinction.

D. O'Neill, R. Starling, A. Kumar, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Gompertz, B. Godson, T. Killestein, M. Pursiainen, on behalf of GOTO collaboration

We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the  EP/WXT alert EP260329b (Song et al; GCN 44142).

We detect the counterpart candidate previously reported (Li et al., GCN 44139; Eyles-Ferris et al., GCN 44144; Magnani et al., GCN 44146; Aryan et al., GCN 44147; Lipunov et al., GCN 44150) with an L-band magnitude of 18.73 ± 0.09 AB mag (+0.74h) at 2026-03-29 11:20:38 UT. We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2026-03-28 11:41:26 (-22.91h) down to a 3-sigma depth of L>20.10 AB mag.

GOTO (https://goto-observatory.org) is a network of telescopes that is
principally funded by the STFC and operated at the Roque de los Muchachos
Observatory on La Palma, Spain, and Siding Spring Observatory in NSW,
Australia, on behalf of a consortium including the University of Warwick,
Monash University, Armagh Observatory & Planetarium, the University of
Leicester, the University of Sheffield, the National Astronomical Research
Institute of Thailand (NARIT), the University of Turku, the University of
Portsmouth, the University of Manchester, the  University of Birmingham
and the Instituto de Astrofisica de Canarias (IAC).

Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (NOIRLab), Benjamin Schneider (LAM), Robert Stein (UMD), Viraj Karambelkar (Columbia), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the field of EP260329b (Song et al., GCN 44142, GCN 44155) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). 

Observations began at 2026-03-30T04:09:30 UTC in the J band (~17.6 hr after the GRB trigger), consisting of 15 x 120 s exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar
 (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565). 

We do not detect a source at the optical counterpart location (Li et al., GCN 44139; Eyles-Ferris et al., GCN 44144; Magnani et al., GCN 44146; Aryan et al., GCN 44147; Lipunov et al., GCN 44150). We obtain the following 5-sigma upper limit: J = 19.3 mag (AB).

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.

F.-F. Song (YNAO, CAS), Guojiong Yang, Wei Chen, Yuan Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP260329b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Song et al., GCN 44142) , and followed by several optical telescopes (Li et al., GCN 44139, Zhu et al., GCN 44140, Eyles-Ferris et al., GCN 44144, Magnani et al., GCN 44146, Aryan et al., GCN 44147, Lipunov et al., GCN 44150). The refined analysis of the WXT data shows that the event started at T0=2026-03-29T10:35:25 (UTC) and lasted for 35 s with a single pulse, before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.51 x 10^20 cm^-2 and a photon index of 1.42 +/- 0.65. The derived average unabsorbed 0.5-4 keV flux is 1.07 (-0.37/+0.56) x 10^(-9) erg/s/cm^2.
 
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-03-29T10:38:50.16 (UTC), about 3 minutes after T0. The exposure time of this observation is 4388 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 181.3131, 4.3505 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT detected a sharp pulse lasting about 200 s. About 400 s after the end of the pulse, the light curve started to rise again. The rising phase was slower than that of the initial pulse and lasted for roughly 800 s, until the observation was interrupted due to Earth occultation. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 1.51 x 10^20 cm^-2 and a photon index of 1.91 +/-0.04. The derived average unabsorbed 0.5-10 keV flux is 1.54 +/-0.05 x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

F.-F. Song (YNAO, CAS), Guojiong Yang, Wei Chen, Yuan Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP260329b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Song et al., GCN 44142) , and followed by several optical telescopes (Li et al., GCN 44139, Zhu et al., GCN 44140, Eyles-Ferris et al., GCN 44144, Magnani et al., GCN 44146, Aryan et al., GCN 44147, Lipunov et al., GCN 44150). The refined analysis of the WXT data shows that the event started at T0=2026-03-29T10:35:25 (UTC) and lasted for 35 s with a single pulse, before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.51 x 10^20 cm^-2 and a photon index of 1.42 +/- 0.65. The derived average unabsorbed 0.5-4 keV flux is 1.07 (-0.37/+0.56) x 10^(-9) erg/s/cm^2.
 
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-03-29T10:38:50.16 (UTC), about 3 minutes after T0. The exposure time of this observation is 4388 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 181.3131, 4.3505 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT detected a sharp pulse lasting about 200 s. About 400 s after the end of the pulse, the light curve started to rise again. The rising phase was slower than that of the initial pulse and lasted for roughly 800 s, until the observation was interrupted due to Earth occultation. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 1.51 x 10^20 cm^-2 and a photon index of 1.91 +/-0.04. The derived average unabsorbed 0.5-10 keV flux is 1.54 +/-0.05 x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-OAFA robotic telescope  [1]  located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP260329b ( EP Team et al., GCN 44142) errorbox  41172 sec after notice time and 46747 sec after trigger time at 2026-03-29 23:35:07 UT, with upper limit up to  17.2 mag. Observations started at twilight.  The observations began at zenith distance = 75 deg. The sun  altitude  is -13.8 deg. 

The galactic latitude b = 65 deg., longitude l = 277 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3219325

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   46777 | 2026-03-29 23:35:07 |         MASTER-OAFA | (12h 05m 25.47s , +04d 20m 43.8s) |   C |    60 | 16.4 |        
   46837 | 2026-03-29 23:35:07 |         MASTER-OAFA | (12h 05m 25.47s , +04d 20m 43.7s) |   C |   180 | 17.2 |  Coadd 
   46852 | 2026-03-29 23:36:21 |         MASTER-OAFA | (12h 05m 25.71s , +04d 21m 49.7s) |   C |    60 | 16.6 |        
   46927 | 2026-03-29 23:37:36 |         MASTER-OAFA | (12h 05m 33.10s , +04d 20m 55.0s) |   C |    60 | 16.7 |        
   47124 | 2026-03-29 23:40:54 |         MASTER-OAFA | (12h 05m 30.36s , +04d 21m 51.8s) |   C |    60 | 16.9 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023,  Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html

A. Aryan, A. Sankar.K, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), M. Nicholl (QUB), M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, W.-J. Hou, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Fulton, K. W. Smith, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:

We observed the field of the fast X-ray transient EP260329b (Song et al., GCN 44142) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations in r-band started at 12:01 UTC on 29th of March 2026 (MJD 61128.501), 1.42 hr after the EP-WXT detection.

We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. Our images were strongly affected by moonlight, and are therefore relatively shallow. In the stacked r-band image at t_mid - t_0 =1.80 hr, we did detect a very faint source (SNR 3.4) at the reported position of the optical counterpart candidate (Wenxiong et al., GCN 44139; Zhu et al., GCN 44140; and Eyles-Ferris et al., GCN 44144).

Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and measured photometry/the 3-sigma upper limit (in the AB system) are as follows:
 
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude    | avg. Seeing | Med. Airmass               
LOT       | r      | 61128.501   | 1.42      | 300 * 8      | 20.19+/-0.14 |1".71        | 1.54     
LOT       | g      | 61128.536   | 2.26      | 300 * 7      | >19.9        |1".50        | 1.34   

Our observations confirm the rapidly fading nature of the optical counterpart candidate.

The presented magnitudes are calibrated using the field stars from ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). The reported magnitudes are not corrected for an expected galactic extinction of A_g = 0.06 mag and A_r = 0.04, in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.

Francesco Magnani (CPPM), Noémie Globus (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of EP 260329b (Song et al., GCN Circ. 44142) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-29 11:21 to 11:41 UTC (from 45.7 to 65.0 minutes after the trigger) and obtained 16 minutes of exposure in the r and z filters, respectively.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

We detected the optical counterpart reported by LCO (Li et al., GCN Circ. 44139, Eyles-Ferris et al., GCN Circ. 44144) and TRT (Zhu et al., GCN Circ 44140), at preliminary magnitudes of:

r = 18.85 +/- 0.08
z = 18.53 +/- 0.13 

Further observations are planned.

We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

R. A. J. Eyles-Ferris (Leicester), A. van Hoof (Radboud), G. Corcoran (UCD), J. Chácon (PUC), A. Martin-Carrillo (UCD), J. Quirola-Vasquez (Radboud),  A. J. Levan (Radboud), F. E. Bauer (SSI and UTA) and P. G. Jonker (Radboud) report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP260329b (Song et al., GCN 44142) with LCO 1m telescopes located at the Siding Spring Observatory (Australia) equipped with the SINISTRO instrument. We obtained 6 x 300 exposures in each of the r and z filters, starting from 2026-03-29 12:03:17 UT (t_mid = 1.74 hr and 1.78 hr post trigger respectively).

We clearly detect the optical counterpart reported by Li et al. (GCN 44139) and Zhu et al. (GCN 44140) in our stacked images and measure the following AB magnitudes calibrated to Pan-STARRS and not corrected for Galactic extinction:

r = 19.84 +/- 0.05
z = 19.38 +/- 0.05

Our measurements are >1 magnitude fainter than those reported by Li et al. (GCN 44139) and Zhu et al. (GCN 44140) indicating the counterpart is fading rapidly.

F.-F. Song (YNAO, CAS), Guojiong Yang, Wei Chen, Yuan Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260329b. The transient triggered EP-WXT (ID: 01709259209) at 2026-03-29T10:36:00 (UTC). The WXT position of the source is R.A. = 181.309 deg, DEC = 4.362 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 181.3122 deg, DEC = 4.3526 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). The FXT position is consistent with the optical counterpart reported by Li et al. (GCN 44139) and Zhu et al. (GCN 44140).

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

F.-F. Song (YNAO, CAS), Guojiong Yang, Wei Chen, Yuan Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:

We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260329b. The transient triggered EP-WXT (ID: 01709259209) at 2026-03-29T10:36:00 (UTC). The WXT position of the source is R.A. = 181.309 deg, DEC = 4.362 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 181.3122 deg, DEC = 4.3526 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). The FXT position is consistent with the optical counterpart reported by Li et al. (GCN 44139) and Zhu et al. (GCN 44140).

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

Wenxiong Li, Runduo Liang (NAOC), Iair Arcav, Ido Keinan (TAU), David Sand (U of Arizona)

We observed the position of EP-WXT trigger 01709259209 with a Las Cumbres 1m telescope at Siding Spring Observatory, Australia, 6 mins after the Einstein Probe WXT trigger. We took 2x300s exposures in the broad optical w-band.
We find an uncataloged source at RA=181.3124, Dec=4.3516 within the EP-WXT error circle and measure the following preliminary photometry calibrated to the r band: 
Epoch 1: MJD 61128.446 Mag 18.7
Epoch 2: MJD 61128.450 Mag 17.1


The presented magnitudes are calibrated using Pan-STARRS sources in the field and are not corrected for Galactic extinction. We identify a faint source at this position in the Legacy Survey data, with 
r=24.08 mag. Additional follow-up observations are encouraged.